1. Field of the Invention
The present invention relates to a beam source suitable for use in a manufacturing process of semiconductor integrated circuits, information storage media such as hard disks, fine optical elements, micromachines, and the like, and more particularly to a beam source for generating a beam having a low energy and a high ion current density from high-density plasma. The present invention also relates to a beam processing apparatus having such a beam source.
2. Description of the Related Art
In recent years, semiconductor integrated circuits, information storage media such as hard disks, fine optical elements, micromachines, and the like have been processed in highly fine patterns. In fields of processing such workpieces, attention has been attracted to use of a high-density energetic beam which is highly linear, i.e., highly directional, and has a relatively large beam diameter. For example, an energetic beam is applied to a workpiece to thereby deposit a film on the workpiece or etch the workpiece.
As beam sources of such energetic beams, there have been used beam generators which generate various kinds of beams including a positive ion beam, a negative ion beam, and a neutral particle beam. The positive ion beam, the negative ion beam, or the neutral particle beam is applied to a desired area of a workpiece from the beam source to thereby locally deposit a film on the workpiece, etch the workpiece, modify a surface of the workpiece, or join or bond parts of the workpiece together.
A positive ion beam source having a dual grid system and a positive ion beam source having a single grid system are disclosed by J. M. E. Harper, J. J. Cuomo, P. A. Leary, G. M. Summa, H. R. Kaufman, and F. J. Bresnock, “Low Energy Ion Beam Etching,” J. Electorochem. Soc., SOLID-STATE SCIENCE AND TECHNOLOGY, 1981. According to Harper et al., a dual grid system can generate ion current densities of 0.5 mA/cm2 with an acceleration voltage of 500 V However, ion current densities of only several hundredths milliamperes per square centimeter can be obtained with an acceleration voltage of 100 V or lower. Therefore, practical ion etching cannot be carried out with an acceleration voltage of 100 V or lower in a dual grid system.
According to Harper et al., a single grid system can generate ion current densities as high as 1 mA/cm2 with an acceleration voltage as small as 20 V However, unless the diameters of beam extraction apertures are comparable to or less than the thickness of a plasma sheath for effective operation, an ion beam cannot be extracted efficiently in parallel trajectories. Since a grid used in the experimental procedure of Harper et al. had 100 lines per inch, the diameters of beam extraction apertures were less than 250 μm. This grid could be used only for 5 to 10 hour operation including several exposures to air.
Another positive ion beam source having a dual grid system is disclosed by Harold R. Kaufman, “Technology of ion beam sources used in sputtering,” J. Vac. Sci. Technol., 15(2), 1978. According to Kaufman, since ion current densities are in inverse proportion to a square of a distance between grid electrodes, it is effective to reduce the distance between the electrodes to enhance ion current densities. However, reduction of the distance between the electrodes is limited to a certain extent because of deflection due to thermal expansion. Kaufman also discloses a method of inserting insulators in some portions of the grid electrodes when a beam diameter is increased. However, this method cannot achieve uniformity of ion current densities on a beam emission surface. Thus, it is difficult to put this method into practice in industrial processes including fine processing on silicon wafers, glass substrates, or the like.
A positive ion beam source having a triple grid system with a diameter of 450 mm is disclosed by Kitamura, “(Direct Current Discharge Type) Ion Engine,” J. Vac. Soc. Jpn., Vol. 45, No. 4, pp. 329–335, 2002. According to Kitamura, beam extraction holes in grid electrodes have diameters of 1 to 2 mm, and the grid electrodes are made of Mo and Ti, which have a low sputtering yield and a small coefficient of thermal expansion. These electrodes are in the form of a bowl in order to release thermal expansion in one direction. Further, the grid electrodes are made of carbon in another example. Carbon has a lower sputtering yield than Mo or Ti, and a coefficient of thermal expansion can be made substantially zero. Thus, it is not necessary to form carbon electrodes into a bowl. According to Kitamura, a potential of a first grid electrode contacting a plasma, which is referred to as a screen electrode, is 1000 V, and a potential of a second grid electrode, which is referred to as an acceleration electrode, is −200 V.
Extraction of ions from a positive ion beam source is discussed by Japanese laid-open patent publication No. 2001-28244, which was filed by the inventors of the present application. In order to efficiently extract ions with a dual grid system, grid electrodes should be designed so that a distance between the grid electrodes is substantially equal to the length of an ion sheath and that the diameters of beam extraction holes in the grid electrodes should be smaller than the distance between the grid electrodes.
The following beam source is disclosed by Japanese laid-open patent publication No. 2001-28244. High-density plasma is generated between two electrodes and spread through a mesh electrode, which is a second upstream electrode, mainly by diffusion. While the electron temperature of the plasma is lowered, downflow plasma is generated between the second electrode and a third electrode. When a negative gas is used, positive-negative ion plasma is generated. When a noble gas such as argon which is unlikely to generate negative ions is used, the plasma density is simply lowered. Ions are accelerated by an electric field produced by a voltage applied between the second and third electrodes. Thus, it is possible to generate high-density plasma. However, the plasma density is lowered so that the sheath length is as long as 1 mm at an upstream side of an electrode for extracting a beam. It is possible to extract a beam from the grid electrode including beam extraction holes having a diameter of 1 mm, which can be readily manufactured. However, this beam source cannot necessarily positively extract a high-density beam.
A negative ion beam source is disclosed by U.S. Pat. No. 5,928,528. This negative ion beam source generates high-density plasma from a negative gas such as halogen or oxygen and converts ions into reactive radicals by recombination of positive ions with electrons while generated ions are transported through a transportation pipe to a plasma chamber. The reactive radicals pass through a large number of holes formed in a plate made of metal. Negative ions are generated by charge exchange between an inner metal surface of the holes and the reactive radicals when the reactive radicals pass through the holes. The negative ions are accelerated by a grid electrode and applied to a workpiece.
However, reactive radicals such as halogen or oxygen have a function to corrode metals. Thus, metals which can be used for the electrodes are practically limited to gold, platinum, silver, ruthenium, rhodium, palladium, osmium, and iridium. Even if these metals are used, corrosion and oxidization cannot completely be eliminated. Thus, these metals are consumable. However, since these metals are expensive, cost of a beam source is further increased when a beam diameter is increased to process wafers having a diameter of 10 inches. Thus, these metals cannot practically be used as industrial materials for the electrodes.
Another negative ion beam source is disclosed by U.S. Pat. No. 4,158,589. This negative ion beam source basically has a dual grid system. In order to accelerate negative ions, a voltage is applied between two grid electrodes having beam extraction holes in the form of slits. A magnetic field of about 1000 gauss (0.1 T) is vertically applied in a direction across the slits of the grid electrodes in a plasma generating chamber, i.e., in a direction in which a beam is emitted. By using a difference between Larmor radii of negative ions and electrons, the negative ions pass through the grid electrode so as to be applied to a workpiece while the electrons are trapped.
A plasma generator described by U.S. Pat. No. 4,158,589 does not generate high-density plasma, i.e., plasma having a positive ion density of 1011 ions/cm3. Although an adjustable voltage can be applied between two electrodes downstream of the plasma generator, a high voltage of 1000 V is applied in the embodiments. Specifically, U.S. Pat. No. 4,158,589 merely discloses that electrons are trapped so as to selectively apply only negative ions to a workpiece. The plasma density is low, and a voltage for accelerating ions is as high as about 1000 V. This beam source produces patterns having a narrow width and may cause damage to the workpiece. Thus, U.S. Pat. No. 4,158,589 does not disclose that a beam is efficiently accelerated at a high speed to process, for example, semiconductor LSIs. More specifically, a beam having a low energy and a high ion current density from high-density plasma is not taught or suggested by U.S. Pat. No. 4,158,589.
A neutral particle beam source is disclosed by U.S. Pat. No. 6,331,701. This neutral particle beam source generates a neutral particle beam having a low energy of 20 to 400 eV from high-density plasma. The beam source has a single grid system including a grid electrode made of aluminum. When an oxygen gas is introduced to generate plasma, a native oxide is formed on the grid electrode made of aluminum. Since a surface of the grid electrode is thus insulated by a dielectric film, ions are accelerated by a self-bias voltage produced between the grid electrode and an acceleration electrode when a high-frequency voltage is applied to the grid electrode. The generated plasma is composed of positive ions and heated electrons which coexist therein. Therefore, when an ion beam having a low energy of 100 eV is extracted from high-density plasma of 1011 ions/cm3, the length of an ion sheath becomes as short as about 0.5 mm. The diameters of beam extraction holes formed in the grid electrode are made smaller than the ion sheath length of, for example, 0.13 mm, because ions cannot efficiently be extracted in parallel without this configuration as described by Harper et al.
However, it is difficult to form fine holes having a diameter of 0.13 mm in the grid electrode. Further, the beam extraction holes should have an aspect ratio as high as about 10 in order to convert ions into a neutral particle beam. Therefore, fine holes are formed in an aluminum plate by dry etching. Thus, the beam extraction holes should have a diameter smaller than the length of the ion sheath in order to efficiently extract a low-energy beam from high-density plasma and should have an aspect ratio of 10 in order to convert ions into a neutral particle beam. Accordingly, the grid electrode has a thickness as thin as 1.6 mm. As a result, the grid electrode cannot maintain a mechanical strength and has a difficulty in handling. In particular, when the diameter of the beam source is increased to 10 inches or more, the grid electrode may be deflected by thermal expansion. Thus, it is difficult to put this neutral particle beam source into industrial practice.
A neutral particle beam processing apparatus is disclosed by Japanese laid-open patent publication No. 2002-289581, which was filed by the inventers of the present application. This neutral particle beam processing apparatus generates a positive-negative ion plasma by pulse modulation of high frequency, then accelerates generated negative ions to a grid electrode, which is referred to as an orifice electrode, with a single or dual grid system, and converts the negative ions into a neutral particle beam when the negative ions pass through beam extraction holes in the grid electrode.
Extraction of an ion beam from plasma is disclosed by Ishikawa, “Ion Source Engineering,” Ionics Co. Ltd., pp. 177–179, 1986 and U.S. Pat. No. 5,827,435. According to Ishikawa and U.S. Pat. No. 5,827,435, plasma is generated by application of a high-frequency voltage. When the application of a high-frequency voltage is interrupted, the electron temperature is lowered so that electrons are attached to a residual gas to form negative ions. Thus, the application of a high-frequency voltage and the interruption of the high-frequency voltage are alternately repeated to generate positive and negative ions. In order to accelerate positive ions and negative ions alternately toward a workpiece or collide the positive ions and negative ions alternately with the workpiece, positive and negative DC bias voltages may alternately be applied to a workpiece, or a high frequency of about 400 kHz may be applied to the workpiece.
For mass production and reduction in cost of semiconductor integrated circuits, fabrication apparatuses for semiconductor integrated circuits should be capable of processing workpieces having larger diameters. When the diameter of a conventional ion beam source is increased so as to generate various kinds of beams including a positive ion beam, a negative ion beam, and a neutral particle beam, the following problems arise.
In order to obtain a high ion current density, an ion beam source having a dual grid system should have a shorter distance between grid electrodes because current densities of ions to be extracted are in inverse proportion to a square of a distance between the grid electrodes and in direct proportion to an extraction voltage to the three halves power as described by Harper et al. In order to obtain a maximum saturation ion current density, the distance between the electrodes should be approximately equal to or slightly shorter than the sheath length, and the diameters of the beam extraction holes should be approximately equal to the distance between the electrodes as described by Ishikawa.
In a conventional ion beam source having a dual grid system, the distance between the electrodes and the diameters of the beam extraction holes can be made long or large because the ion beam source has a low plasma density. In generally used plasma which is composed of positive ions and heated electrons which coexist therein, when the extraction voltage is as low as 100 V and the high-density plasma has a positive ion density of 1011 ions/cm3, the sheath length becomes as short as about 0.5 mm.
The sheath length s is expressed by
      s    ⁡          [      m      ]        =      0.585    ⁢                            λ          D                ⁡                  (                                    2              ⁢                              V                0                                                    kT              e                                )                            3        4            where Te is the electron temperature, λD is the Debye length, and V0 is the sheath potential [V]. The Debye length λD is expressed by
            λ      D        ⁡          [      m      ]        =                                          ɛ            0                    ⁢          k          ⁢                                          ⁢                      T            e                                                e            2                    ⁢                      n            i                                =          7.43      ×              10        3            ⁢                                                  kT              e                        ⁢                                                  [            eV            ]                                              n              i                        ⁢                                                  [                          m                              -                3                                      ]                              where ε0 is the permittivity of empty space, and ni is the plasma density.
It is practically difficult to provide an ion beam source which can obtain a high ion current density from this plasma because the diameters of the beam extraction holes and the distance between the electrodes become extremely small or short. Even if such an ion beam source can be provided, the grid electrodes are deflected because they are as thin as at most 0.5 mm. In order to solve such drawbacks, the grid electrodes may be fixed at some portions by pins of ceramic insulators. As described by Harper et al., while such a method can be applied to an ion engine, the uniformity of a beam becomes worse in a case where such a method is employed for semiconductor fabrication apparatuses. There has been known to curl electrodes made of molybdenum or titanium in the form of a bowl in order to maintain the distance between the electrodes by deflection of the electrodes in the same direction. However, with this method, when beam extraction holes in the grid electrodes have a high aspect ratio, e.g., a diameter of 0.13 mm and a length of 1.6 mm, a beam cannot be emitted in parallel without divergence because the grid electrodes are curled. The drawbacks on deflection can be solved if grid electrodes are made of a graphite material, which has a small coefficient of linear expansion. However, since the graphite material is frangible, such electrodes may be broken in handling. Further, when the beam extraction holes have a small diameter, it is difficult to align the beam extraction holes of the respective grid electrodes with each other.
An ion beam source having a single grid system disclosed by Harper et al. can achieve a saturation ion current density with a low extraction voltage. For this purpose, the ion extraction holes should have a diameter smaller than the sheath length. When the plasma density is as high as about 1011 ions/cm3, the sheath length is as short as about 0.5 mm. Therefore, the ion extraction holes should have a diameter of about 0.1 mm. In order to efficiently extract ions, the electrode should be thin in the same manner as the diameter of the ion extraction holes. However, a thin electrode has a short lifetime because it is sputtered by accelerated ions. In an acceleration device disclosed by U.S. Pat. No. 6,331,701, electrodes are similarly sputtered by accelerated ions.
As described above, the conventional ion beam sources cannot generate an ion beam having a low energy of at most 500 V, preferably at most 200 V, and a high ion current density from high-density plasma. Therefore, in industrial processes, a high etching rate cannot practically be achieved with use of an ion beam having a high ion current density and a low energy.
Further, a reactive ion etching (RE) process has been widely employed in various industrial fields of fine processing. In particular, positive-negative ion plasma as described by U.S. Pat. Nos. 5,928,528 and 5,827,435 is advantageous in that charge build-up damage and microloading effect can be prevented unlike plasma composed of positive ions and electrons. However, a grid electrode is not disposed between plasma and a workpiece unlike a beam source. Thus, a workpiece is exposed directly to the plasma. Therefore, an undesired film adversely deposits on a surface or a side surface of the workpiece due to unnecessary exposure of the workpiece to radicals, or a vacuum ultraviolet (VUV) emitted from the plasma is applied to the workpiece so as to cause semiconductor devices formed on the workpiece to be damaged.